Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. Once plants are yellow, yield loss has already occurred. The treatment thresholds are 20 per 100 sweeps on alfalfa 3 inches or less in height, 50 per 100 sweeps in 4-6 inch tall alfalfa and 100 per 100 sweeps in 7-11 inch tall alfalfa.
Field Corn
In recent years, we have seen an increase in stinkbug damage to developing corn ears, especially when fields are adjacent to small grain fields. We are continuing to survey fields to evaluate the extent of the damage this season. Information from the University of Georgia, where they have experienced problems, indicates that:
(a) Corn is most susceptible to stink bug injury during ear formation before tasseling.
(b) Bugs will feed through the sheath, causing a dead spot on the ear. As the ear expands it becomes distorted and curves, usually outward.
(c) Feeding during silking and pollen shed also will kill kernels on the ear. Once the ear has elongated, stink bug feeding during the blister and milk stages blasts individual kernels usually causing them to abort.
(d) Although we have not developed thresholds for our area, the following thresholds are used in the South : 25% infested plants (1 bug per 4 plants) as a threshold during ear elongation to pollen shed and 50% infested plants (1 bug per 2 plants) during the later part of pollen shed and blister/milk stage.
(e) Initially stink bugs tend to be more prevalent on the field edge, so only a perimeter spray may be needed.
Soybeans
Be sure to sample fields in the seedling stage for bean leaf beetles, grasshoppers and spider mites. Grasshopper populations are starting to increase, especially in no-till fields. As barley and wheat are harvested and soybeans are planted, these fields will be susceptible to attack and grasshopper feeding can often cause stand loss. If stand reductions are occurring from plant emergence to the second trifoliate, a treatment should be applied. Although no precise thresholds are available, a treatment may be needed if you find one grasshopper per sweep and 30% defoliation from plant emergence through the pre-bloom stage. Numerous products are labeled for grasshopper control including a number of pyrethroids, dimethoate, Lorsban, Orthene 97 and Sevin XLR. Be sure to check all labels carefully before combining insecticides and herbicides since there are a number of restrictions on the labels.
Continue to watch carefully for spider mites. We are finding fields with economic levels of mites, both on field edges and in some cases in field interiors – so be sure to scout the entire field to make a treatment decision. Labeled materials include dimethoate, Lorsban, Hero (zeta-cypermethrin + bifenthrin) as well as a number of stand alone bifenthrin products (not all may be labeled so be sure to check the label). All of these products need to be applied before mites explode. Be sure to read the labels for use rates and restrictions – there is a limit on the number of applications as well as the time between applications on all of the materials labeled for spider mite control.